Quota

Quota backend specifies the method how Dovecot keeps track of the current quota usage. They don't (usually) specify users' quota limits, that's done by returning extra fields from userdb. There are different quota backends that Dovecot can use:

Configuration

Quota recalculation

If your quotas are out of sync, you can use doveadm quota recalc command to recalculate them.

Quota and Trash mailbox

Standard way to expunge messages with IMAP works by:

Marking message with \Deleted flag

Actually expunging the message using EXPUNGE command

Both of these commands can be successfully used while user's quota is full. However many clients use a "move-to-Trash" feature, which works by:

COPY the message to Trash mailbox

Mark the message with \Deleted

Expunge the message from the original mailbox.

(Maybe later expunge the message from Trash when "clean trash" feature is used)

If user is over quota (or just under it), the first COPY command will fail and user may get an unintuitive message about not being able to delete messages because user is over quota. The possible solutions for this are:

Disable move-to-trash feature from client

You can create a separate quota rule ignoring Trash mailbox's quota. Note that this would allow users to store messages infinitely to the mailbox.